 As we saw in Section 2.1, the problem of finding the tangent line to a curve and the problem of finding the velocity of an object both involve finding the same type of limit.  This special type of limit is called a derivative. LIMITS AND DERIVATIVES

2.7 Derivatives and Rates of Change LIMITS AND DERIVATIVES In this section, we will see that: the derivative can be interpreted as a rate of change in any of the sciences or engineering.

 If a curve C has equation y = f (x) and we want to find the tangent line to C at the point P(a,f (a)), then we consider a nearby point Q(x,f (x)), where, and compute the slope of the secant line PQ: TANGENTS

 Then, we let Q approach P along the curve C by letting x approach a.  If m PQ approaches a number m, then we define the tangent t to be the line through P with slope m.  This m amounts to saying that the tangent line is the limiting position of the secant line PQ as Q approaches P. TANGENTS

 The tangent line to the curve y = f (x) at the point P(a,f (a)) is the line through P with slope provided that this limit exists.  An equation of this tangent line is given by TANGENTS 1. Definition

 We sometimes refer to the slope of the tangent line to a curve at a point as the slope of the curve at the point.  The idea is that, if we zoom in far enough toward the point, the curve looks almost like a straight line. TANGENTS

 The figures illustrate this procedure for the curve y = x 2.  The more we zoom in, the more the parabola looks like a line.  In other words, the curve becomes almost indistinguishable from its tangent line. TANGENTS

 There is another expression for the slope of a tangent line that is sometimes easier to use.  If h = x - a, then x = a + h and so the slope of the secant line PQ is: TANGENTS

 In the figure, the case h > 0 is illustrated and Q is to the right of P.  If it happened that h < 0, Q would be to the left of P. TANGENTS

 Notice that, as x approaches a, h approaches 0 (because h = x - a).  So, the expression for the slope of the tangent line in Definition 1 becomes: TANGENTS 2. Definition

 Find an equation of the tangent line to the hyperbola y = 3/x at the point (3, 1).  Let f(x) = 3/x  Then, the slope of the tangent at (3, 1) is: TANGENTS Example 2

 Therefore, an equation of the tangent at the point (3, 1) is  This simplifies to x + 3y – 6 = 0  The hyperbola and its tangent are shown in the figure. TANGENTS Example 2

 In Section 2.1, we investigated the motion of a ball dropped from the CN Tower and defined its velocity to be the limiting value of average velocities over shorter and shorter time periods. VELOCITIES

 In general, suppose an object moves along a straight line according to an equation of motion s = f (t)  s is the displacement (directed distance) of the object from the origin at time t.  The function f that describes the motion is called the position function of the object. VELOCITIES

 In the time interval from t = a to t = a + h, the change in position is f (a + h) – f (a) VELOCITIES

 The average velocity over this time interval is  This is the same as the slope of the secant line PQ in the second figure. VELOCITIES

 Now, suppose we compute the average velocities over shorter and shorter time intervals [a, a + h].  In other words, we let h approach 0. VELOCITIES

 As in the example of the falling ball, we define the velocity (or instantaneous velocity) v(a) at time t = a to be the limit of these average velocities:  This means that the velocity at time t = a is equal to the slope of the tangent line at P. VELOCITIES 3. Definition

 Limits of the form arise whenever we calculate a rate of change in any of the sciences or engineering, such as a rate of reaction in chemistry or a marginal cost in economics.  Since this type of limit occurs so widely, it is given a special name and notation. DERIVATIVES

 The derivative of a function f at a number a, denoted by f’(a), is: if this limit exists. DERIVATIVES 4. Definition

 If we write x = a + h, then we have h = x – a and h approaches 0 if and only if x approaches a.  Therefore, an equivalent way of stating the definition of the derivative—as we saw in finding tangent lines—is: DERIVATIVES 5. Definition

 We defined the tangent line to the curve y = f (x) at the point P(a,f (a)) to be the line that passes through P and has slope m T given by Equation 1 or 2.  Since, by Definition 4, this is the same as the derivative f’(a), we can now say the following.  The tangent line to y = f (x) at (a,f(a)) is the line through (a,f (a)) whose slope is equal to f ’(a), the derivative of f at a. DERIVATIVES

 If we use the point-slope form of the equation of a line, we can write an equation of the tangent line to the curve y = f (x) at the point (a, f (a)): DERIVATIVES

 Find the derivative of f (x) = - x 2 +4x +2 at the number a.  From Definition 4, we have: DERIVATIVES Example 4

 Therefore the derivative of f (x) = - x 2 +4x +2 at the number a is given by the expression  That is, the slope of the tangent line to the graph of f (x) = - x 2 +4x +2 at the number a is given by  In a previous hand out (In class 5) we studied this function in detail. Here is a review. DERIVATIVES Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ).  For this, we used the approximation where In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ). In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ).  Since a is arbitrary, this table specifies a new function, namely the one that assigns to every point a in the domain of f the number m T = f '(a), the slope of the tangent line at a. In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ). In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ). In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ). In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ). In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ). In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ). In class Assignment 5 Example 4

 Problem: Find the slope of the tangent line to the graph of the function f (x) = - x 2 +4x +2 at the points x = -1, 0, 1, 2, 3, 4, and 5 (these are the different values of a ). In class Assignment 5 Example 4

 Suppose y is a quantity that depends on another quantity x.  Thus, y is a function of x and we write y = f (x)  If x changes from x 1 to x 2, then the change in x (also called the increment of x) is denoted by:  The corresponding change in y is: RATES OF CHANGE

 The difference quotient is called the average rate of change of y with respect to x over the interval [x 1,x 2 ] and can be interpreted as the slope of the secant line PQ in the figure. RATES OF CHANGE

 By analogy with velocity, we consider the average rate of change over smaller and smaller intervals by letting x 2 approach x 1 and, therefore, letting approach 0.  The limit of these average rates of change is called the (instantaneous) rate of change of y with respect to x at x = x 1. RATES OF CHANGE

 This is interpreted as the slope of the tangent to the curve y = f (x) at P(x 1, f (x 1 )):  Instantaneous rate of change equals  We recognize this limit as being the derivative f '(x 1 ) RATES OF CHANGE

 We know that one interpretation of the derivative f '(a) is as the slope of the tangent line to the curve y = f (x) when x = a.  We now have a second interpretation.  The derivative f '(a) is the instantaneous rate of change of y = f (x) with respect to x when x = a. RATES OF CHANGE

 The connection with the first interpretation is that, if we sketch the curve y = f (x), then the instantaneous rate of change is the slope of the tangent to this curve at the point where x = a. RATES OF CHANGE

 This means that:  When the derivative is large (and therefore the curve is steep, as at the point P in the figure), the y-values change rapidly.  When the derivative is small, the curve is relatively flat and the y-values change slowly. RATES OF CHANGE

 In particular, if s = f (t) is the position function of a particle that moves along a straight line, then f’(a) is the rate of change of the displacement s with respect to the time t.  In other words, f’(a) is the velocity of the particle at time t = a.  The speed of the particle is the absolute value of the velocity, that is, |f’(a)|. RATES OF CHANGE

 In the next example, we discuss the meaning of the derivative of a function that is defined verbally. RATES OF CHANGE

 A manufacturer produces bolts of a fabric with a fixed width. The cost of producing x yards of this fabric is C = C (x) dollars. a. What is the meaning of the derivative C ′(x)? What are its units? b. In practical terms, what does it mean to say that C ′(1,000) = 9? c. Which do you think is greater, C ′(50) or C ′(500)? What about C ′(5,000)? RATES OF CHANGE Example 6

 The derivative C′(x) is the instantaneous rate of change of C with respect to x.  That is, C′(x) means the rate of change of the production cost with respect to the number of yards produced.  Economists call this rate of change the marginal cost. RATES OF CHANGE Example 6 a

 As the units for C ′(x) are the same as the units for the difference quotient.  Since is measured in dollars and in yards, it follows that the units for C′(x) are dollars per yard. RATES OF CHANGE Example 6 a

 The statement that C′(1,000) = 9 means that, after 1,000 yards of fabric have been manufactured, the rate at which the production cost is increasing is $9/yard.  When x =1,000, C is increasing 9 times as fast as x. RATES OF CHANGE Example 6 b

 Since is small compared with x =1,000, we could use the approximation and say that the cost of manufacturing the 1,000th yard (or the 1,001st) is about $9. RATES OF CHANGE Example 6 b

 The rate at which the production cost is increasing (per yard) is probably lower when x = 500 than when x = 50 (the cost of making the 500th yard is less than the cost of the 50th yard) due to economies of scale.  The manufacturer makes more efficient use of the fixed costs of production.  Therefore, RATES OF CHANGE Example 6 c

 However, as production expands, the resulting large-scale operation might become inefficient and there might be overtime costs.  Thus, it is possible that the rate of increase of costs will eventually start to rise.  So, it may happen that RATES OF CHANGE Example 6 c

 In the following example, we estimate the rate of change of the national debt with respect to time.  Here, the function is defined not by a formula but by a table of values. RATES OF CHANGE

 Let D(t) be the US national debt at time t.  The table gives approximate values of this function by providing end-of-year estimates, in billions of dollars, from 1980 to  Interpret and estimate he value of D′(1990). RATES OF CHANGE Example 7

 The derivative D′(1990) means the rate of change of D with respect to t when t =1990, that is, the rate of increase of the national debt in RATES OF CHANGE Example 7

 By Equation 5,  So, we compute and tabulate values of the difference quotient as follows. RATES OF CHANGE Example 7

 From the table, we see that D’(1990) lies somewhere between and billion dollars per year.  Here, we are making the reasonable assumption that the debt did not fluctuate wildly between 1980 and RATES OF CHANGE Example 7

 We estimate that the rate of increase of the national debt in 1990 was the average of these two numbers—namely billion dollars per year. RATES OF CHANGE Example 7

 Another method would be to plot the debt function and estimate the slope of the tangent line when t = RATES OF CHANGE Example 7

 In Examples 3, 6, and 7, we saw three specific examples of rates of change :  The velocity of an object is the rate of change of displacement with respect to time.  The marginal cost is the rate of change of production cost with respect to the number of items produced.  The rate of change of the debt with respect to time is of interest in economics. RATES OF CHANGE

 Here is a small sample of other rates of change:  In physics, the rate of change of work with respect to time is called power.  Chemists who study a chemical reaction are interested in the rate of change in the concentration of a reactant with respect to time (called the rate of reaction).  A biologist is interested in the rate of change of the population of a colony of bacteria with respect to time. RATES OF CHANGE

 In fact, the computation of rates of change is important in all the natural sciences, in engineering, and even in the social sciences.  All these rates of change are derivatives and can therefore be interpreted as slopes of tangents.  This gives added significance to the solution of the tangent problem. RATES OF CHANGE

 Whenever we solve a problem involving tangent lines, we are not just solving a problem in geometry.  We are also implicitly solving a great variety of problems involving rates of change in science and engineering. RATES OF CHANGE